Estimate gravel, collecting pipe, filter-wrap area and drop for one straight French-drain run. For a 50-foot run with a 12-inch-wide, 18-inch-deep gravel envelope, the approximate 4-inch pipe model and 10% allowance give 2.878 cubic yards of gravel. A full rectangular wrap with our 12-inch top-overlap allowance is 300 square feet, before end closures or seams. This is a quantity model, not a drainage design.
Build a drainage budget from the full scope
Price the selected gravel order, collecting pipe and suitable filter fabric separately. The straight-run outputs are not a complete shopping list: outlet pipe, fittings, end closures, fabric seams, excavation and surface restoration are outside this model. Confirm the actual drainage design and outlet before using these quantities to request a quote.
Our home-improvement planning guide helps you compare the same measured scope across suppliers and installers. Record product specifications, whole rolls or order increments, delivery, excavation, disposal, labor and restoration. Mark each item as included, excluded or unknown, and do not add another material allowance to a quote that already includes it.
The quantity formulas
Envelope volume (cu ft) = length (ft) × width (in) / 12 × gravel depth (in) / 12.
Approximate pipe displacement (cu ft) = length × π × (diameter / 2)² / 144.
Gravel (cu yd) = (envelope volume – pipe displacement) × (1 + allowance / 100) / 27.
Full-wrap strip width (in) = 2 × gravel depth + 2 × width + 12.
Fabric area (sq ft) = length × strip width / 12.
Drop (in) = length (ft) × 12 × slope / 100. These are our geometry and quantity formulas; the 12-inch overlap is our planning assumption, not a universal fabric instruction.
The fabric must cover bottom, both sides and top before overlap. Both the bottom and top consume one trench width. At 12 inches wide and 18 inches deep, that is 12 + 18 + 12 + 18 + 12 overlap = 72 inches, or 6 feet of strip width. This corrects the former 5-foot strip model, which omitted one width. End closures, splices, irregular trench walls and cutting waste are additional.
Gravel envelope versus total excavation
Enter the depth of the rectangular volume to be filled with stone, not the full excavation depth if part is reserved for soil, sod or another cover. Example: an excavation 18 inches deep with 4 inches of soil above a 14-inch stone zone uses 14 inches in this form. The pipe must fit inside that zone with the bedding and surrounding stone required by the design. The form only checks diameter against width and depth; that is not enough to approve clearance.
The NDS guide describes fabric-lined trenches, gravel, perforated pipe directed toward an outlet, and additional fabric plus topsoil/sod above the gravel.1 Its general guidance is not a site-specific section detail. A product with prepackaged aggregate, such as EZflow, does not use this loose-gravel model unchanged.
Gravel and fabric chart by run length
Our calculated chart assumes a 12-inch width, 18-inch stone depth, approximate 4-inch pipe displacement, 10% allowance and 12-inch top overlap. Fabric includes a full rectangular wrap, but no end/seam allowance. None of these example dimensions is a required design.
| Run | Gravel with 10% | Full-wrap fabric | Drop at 1% |
|---|---|---|---|
| 10 ft | 0.576 cu yd | 60 sq ft | 1.2 in |
| 25 ft | 1.439 cu yd | 150 sq ft | 3 in |
| 50 ft | 2.878 cu yd | 300 sq ft | 6 in |
| 75 ft | 4.317 cu yd | 450 sq ft | 9 in |
| 100 ft | 5.756 cu yd | 600 sq ft | 12 in |
Three checked examples
1. 50-foot run, 12 by 18-inch stone envelope
Envelope: 75.000000 cu ft. The approximate 4-inch pipe subtracts 4.363323 cu ft. Net stone volume: 70.636677 cu ft. With 10% allowance: 2.877791 cu yd. Full-wrap width: (2 × 18 + 2 × 12 + 12) / 12 = 6.000000 ft; area: 50 × 6.000000 = 300.00 sq ft. At 1% slope: 50 × 12 × 1/100 = 6 inches of drop. Pipe model, overlap and allowance are example inputs.
2. 50-foot run, 12 by 14-inch stone envelope
Envelope: 58.333333 cu ft. The approximate 4-inch pipe subtracts 4.363323 cu ft. Net stone volume: 53.970010 cu ft. With 10% allowance: 2.198778 cu yd. Full-wrap width: (2 × 14 + 2 × 12 + 12) / 12 = 5.333333 ft; area: 50 × 5.333333 = 266.67 sq ft. At 1% slope: 50 × 12 × 1/100 = 6 inches of drop. The second example could represent an 18-inch excavation with 4 inches reserved for cover; this is a quantity illustration, not an approved section.
3. 100-foot run, 18 by 24-inch stone envelope
Envelope: 300.000000 cu ft. The approximate 6-inch pipe subtracts 19.634954 cu ft. Net stone volume: 280.365046 cu ft. With 5% allowance: 10.903085 cu yd. Full-wrap width: (2 × 24 + 2 × 18 + 12) / 12 = 8.000000 ft; area: 100 × 8.000000 = 800.00 sq ft. At 2% slope: 100 × 12 × 2/100 = 24 inches of drop. Pipe model, overlap and allowance are example inputs.
Pipe diameter and weight are estimates to verify
The dropdown subtracts a smooth cylinder of either 4 or 6 inches in diameter. Nominal pipe size is not guaranteed to equal actual outside diameter; corrugations and fittings further change displacement. Confirm the outside profile of your chosen product. The form does not select pipe capacity, perforation arrangement, bedding or minimum cover. Pipe length is only the collecting run; add solid outlet runs, bends, couplers, branches, cleanouts and cut allowance separately.
There is no universal tons-per-yard value here. Ask the supplier for the bulk density of the exact washed stone and moisture condition, then multiply cubic yards by short tons per cubic yard. For illustration only, 2.877791 cubic yards at an assumed 1.4 short tons/yard would weigh 4.028907 short tons. That 1.4 is not a verified density for your order. Use the gravel calculator to explore a supplier-provided density.
Filter fabric is not automatically weed barrier
NDS describes fabric as keeping dirt from mixing with gravel while allowing percolation.1 Choose a drainage filter suited to the soil and intended assembly, using the product/design requirements for opening size, permeability, strength and installation. A retail weed-barrier listing alone does not establish drainage-filter suitability. This page no longer points to a weed barrier as if it were an approved drain wrap.
Strip width and total area are different shopping checks. A 6-foot required strip is not automatically satisfied by a 5-foot roll even if the roll has enough square feet. If seams or multiple strips are used, follow the approved seam/overlap detail and account for the added material. Our fixed 12-inch closure overlap is not a substitute for that detail.
Slope is exact math, not a guaranteed design
A 1% slope is 0.01 foot of fall per foot, or exactly 0.12 inch per foot. Over 50 feet, that is 6 inches. One inch over 8 feet is about 1.0417%, not exactly 1%. NDS gives the grade formula (rise / run) × 100.2 Its quick-review sheet describes minimum slope guidance and notes that corrugated pipe can require more slope; critical applications need an engineer or architect.2
The general NDS French-drain article discusses a 1% guideline and different advice for corrugated pipe.1 Those documents do not establish one slope for every site. The 1% default is a calculation example. Zero is accepted for arithmetic but does not certify a functioning gravity drain. Identify a suitable permitted outlet and check elevations before digging; the slope result does not calculate cover depth along uneven ground or discharge capacity.
Before excavation
Contact 811 using the rules for your state, wait for the required responses and markings, and follow safe-excavation requirements. The Illinois JULIE FAQ explains that utilities locate the lines they own or maintain, not all private lines; examples include irrigation and secondary electric runs.3 That is Illinois guidance, not a nationwide waiting-time rule. Check private utilities as well. Do not assume one call marks every buried line.
Water problems near foundations, retaining walls or underground rooms need site-specific assessment. Do not excavate beside structural footings based on this calculator. Confirm outlet permission, setbacks, existing services and the effect on neighboring property with a qualified drainage professional when uncertain.
Metric inputs
The toggle converts run length to meters and cross-section inputs to centimeters. Pipe model options remain in inches as labeled. Equivalent inputs preserve the quantities; the default 50 feet becomes 15.24 meters, width 30.48 cm and stone depth 45.72 cm. Slope is a percentage in either unit system. Metric fabric output is area, not roll width; check the indicated strip-width model separately.
FAQ
Why does the fabric formula use width twice?
A fully wrapped rectangular stone envelope has a bottom and a top, each one width, plus two sides. The fixed overlap is then added. The old one-width formula omitted the top or bottom.
Does this tell me how deep to dig?
No. It estimates a selected stone envelope. Required elevation, cover, slope, outlet, soil, utilities and structure determine the actual excavation design.
Is more gravel always better?
No. Extra volume does not establish drain capacity, correct soil filtration or a usable outlet. This calculator is a quantity aid, not proof that the drainage plan will solve the water problem.
Related guides
How much gravel for a French drain? | Drainage stone size and type | Gutter calculator
Sources
- NDS: French Drain Installation Guide (general layout, fabric, pipe, outlet and cover guidance)
- NDS: Principles of Exterior Drainage – Quick Review (grade formula, pipe slope caveats and critical design advice)
- JULIE: Excavator FAQs (Illinois locate responses and limits on locating private utilities)
What you’ll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Perforated drain pipe – carries water along the trench.
- Drainage filter fabric selected for the soil – follow the filter and assembly specification; generic weed barrier is not automatically suitable.
- Washed drainage stone – see the gravel calculator.
- Shovel or trenching tool – for digging.
- Level or string line – to hold a steady slope.
- Pipe outlet or pop-up fitting – where the water leaves.
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Estimates only. Confirm quantities with your supplier before you order.